The invention relates to a Wheatstone bridge comprising conventionally interconnected bridge elements, consisting of a spin-valve system, as well as a method for producing the same.
Wheatstone bridges of this kind are preferably used as sensors for measuring small magnetic fields and as non-contactingly measuring angle sensors.
According to the prior art magneto-resistive strip conductors are used to measure magnetic fields with respect to magnitude and direction, whereby these strip conductors are anisotropic as to their magneto-resistive properties and, inter alia, interconnected in a Wheatstone bridge manner (refer to, for example, DD 256 628, DE 43 17 512 A1). The magneto-resistive strip conductors, which are employed thereby, exhibit anisotropic resistance chances with respect to an external magnetic field, which changes being a desirable property when the intended application, for example, is as a selsyn. However, such strip conductors, for example, on the basis of permalloy only show maximal resistance chances of about 2-3%, so that a comparatively high expenditure as to electronics and manufacture has to be invested.
Furthermore, materials and structures, respectively, have become known which include a so-called giant magneto-resistance (refer to, for example, S. P. P. Parkin et al., Oscillatory magnetic exchange coupling through thin copper layers, Phys. Rev. Lett., Vol. 66, pp. 2152, 1991 and R. von Helmolt et al., Giant Negative Magnetoresistance in Perovskite like La2/3Ba1/3MnOx. Ferromagnetic Films, Phys. Rev. Lett., Vol. 71, No. 14, pp. 2331, 1993). This class of materials and structures, respectively, show magneto-resistive resistance effects, which surpass the generally used magneto-resistive materials by one to several order/s of size. The disadvantage of these materials for the intended applications consists in that they do not exhibit any anisotropic resistance effect.
Magneto-resistive sensors are realized in known manner in the form of Wheatstone bridges in order to minimize the influence by ambience on the measuring signal, such as changes in temperature, or to entirely suppress these influences. It is, however, a condition for the setup of such Wheatstone bridges that adjacent bridge branches of a half bridge will, with respect to a magneto-resistive resistance change, oppositely behave under the influence of an external magnetic field. When employing anisotropic magnetic materials such as permalloy (Ni81Fe19) which is used in typical MR-sensors, this can be realized in a comparatively simple manner in that by orienting two magneto-resistive strip conductors at right angles to each other within a half bridge or by using Barber poles, the direction of the current flowing in the magneto-resistive bridge branches is differently impressed. In the case of isotropic resistance systems such as, for example, systems with a Giant Magneto-Resistive effect, the previously used approaches do not deliver any satisfactory solution. In DE 195 32 674 C1, for example, there is shown with respect to rotation angle sensors a possible approach for anti-ferromagnetic coupled multi-layer systems or for layer systems with a very great magneto-resistive resistance effect. Therein a change of magnetic fields affecting adjacent bridge branches is achieved by a conveniently formed geometry of soft-magnetic antenna geometries operating as magnetic collectors. Though this approach creates the desired effect, it includes additional structures and complicated structuring processes and it is only suited for rotation angle measurements.
Furthermore, there are layer systems known with a so-called spin-valve effect which are preferably used for detecting small fields, but also for angle detection (refer to, for example, DE 43 01 704 A1). These layer systems have in common that they consist of magnetic single layers in which, in an ideal case, one sensor layer is magnetically slightly rotatable and a bias layer is magnetically immobile. Up to now these layers can only operate as single magneto-resistive strip sensors. Though comparatively high signals are obtainable with such sensors, the measuring signals are affected by all further interference sources, such as temperature changes.
A solution for avoiding such problems is described in DE 196 49 265 A1, in which a GMR-sensor with a Wheatstone bridge is described, in which spin-valve systems are used for the individual bridge elements. This solution, however, requires a comparatively complicated layout for the Wheatstone bridges arranged on comparatively large chip surface areas (1 . . . 4 mm2). Due to this strictly required layout, a further miniaturization is not possible with this solution.
The layer setup of a spin-valve system can be designed as a GMR-layer system (under use of Giant Magneto-Resistive materials) or as a TMR-layer system (tunnel layer system). Thereby the layer system is comprised of at least one anti-ferromagnetic layer, one ferromagnetic layer pinned through the anti-ferromagnetic layer via a so-called exchange bias, whereby the ferromagnetic layer itself is a component of a so-called artificial anti-ferromagnet (AAF), at least one flux conducting layer and, arranged between these ferromagnetic layers, one conductive layer for the GMR-layer systems or for the oxidic layer of the tunnel arrangement. Thereby a magneto-resistive sensor system of at least two sensor elements can be formed by this layer setup. When used in applications, these sensor elements are typically assembled to Wheatstone bridges.
The general setup of magneto-resistive sensors, their mode of operation and their applications can be read from “Sensors—A comprehensive Survey” (editor: W. Göpel et al.), VCH publishing company Weinheim, Vol.5: Magnetic Sensors (editor: R. Boll et al.), 1989, Chapter 9: Magneto-resistive Sensors, p. 341 to 378. The represented sensors show an anisotropic magneto-resistive effect. Furthermore, it can be learned from the reference how to form sensor bridges which, for example, can be used for manufacturing 360° angle detectors. Such bridges can also be setup with sensors which are in accordance with the layer setup mentioned hereinbefore. Also here it is necessary that two sensors out of the four, which make up the bridge, are oppositely oriented to the other two with respect to the bias layer magnetization in order to obtain respective signals across the entire angular range. This is also necessary with sensors operating on the basis of a magnetic tunnel effect or with spin-valve transistors.
The setting of the bias magnetization direction (BMD) is generally carried out by applying a homogeneous magnetic field in the course of depositing a magnetic layer system onto a 3-6″ Si-wafer. This results in all over the same BMD. The Patent DE 198 30 343 C1 discloses, how an anti-parallel orientation of the BMD can be achieved by a suitable selection of the layers, just as in the case where combinations of anti-ferromagnetic layers as well as layer systems which are designed as artificial anti-ferromagnets are utilized. According to this reference, this has been achieved in that, in order to enable a local anti-parallel orientation of the magnetization of the bias layers after manufacturing the AAF-system, the symmetry of the AAF-system is locally affected in such a manner that the affected and non-affected ranges of the layer setup exhibit a different behavior in a homogeneous magnetic field. Consequently, this proposal abandons an identical layer setup for all the sensor elements and for all ranges, respectively, which are intended to form sensor elements. This generally creates parasitic asymmetries with respect to resistance and, above all, to the temperature coefficient of the resistance which harmfully affects the operation behavior.
A second possibility consists in setting up the Wheatstone bridge in hybrid form in such a manner that the bridge branches consist of elements being geometrically rotated by 180° in order to achieve an anti-parallel positioning of the BMD. The first mentioned method requires suitable additional layers within the AAF having suitable properties. The last mentioned method means a considerable additional expenditure in the manufacture of Wheatstone bridges, namely an additional assembly expenditure as well as an additional expenditure for the wiring, which involves apart from higher cost, a deterioration of the reliability.